In general, polyesters, particularly polyethylene terephthalates, possess good chemical stability, mechanical strength, heat resistance, durability and chemical resistance; and therefore, they have been widely used in manufacturing condensers, medical articles, photographic films, packaging and labelling materials and, particularly, magnetic recording media.
In order to provide a film manufactured from such polyesters with an excellent runnability, handling property and surface properties, inorganic slip agents such as calcium carbonate, silica, alumina or kaoline are incorporated into the polyester film to form minute protuberances on the surface thereof. Among these inorganic slip agents, calcium carbonates control efficiently the surface properties of the film; and alumina provides an excellent scratch resistance to the film. However, they have a low affinity to the polyester, which may lead to the formation of voids during a draw process of the film. In a further processing of the film, such voids may cause the surface to abrade and the particles to be separated therefrom. Such a polyester film, when used as a magnetic tape, may be easily damaged.
As one of conventional methods for improving the scratch resistance of a polyester film, Japanese Patent Laid-open Publication No. 90-185533 discloses a method for improving the surface properties and scratch resistance of a polyester film by employing mixtures of a heavy calcium carbonate and alumina.
To diminish the abrasion of the polyester film caused by a lack of affinity between the polyester and the particles, Japanese Patent Laid-open Publication No. 90-11640 describes the addition of organic particles, such as silicon polyamide, crosslinked styrene-divinylbenzene copolymer, crosslinked polyester or Teflon, along with inorganic particles, such as calcium carbonate, into a polyester to produce a polyester film with improved surface properties and scratch resistance.
Although the films prepared by these methods may possess good surface properties and scratch resistance, however, they still have the problem of a low affinity between the organic and inorganic particles and to the polyester, and, therefore, the abrasion resistance may decline, allowing the particles to break away from the film on high running conditions.